Brazing sheets have been conventionally used as material for tubes in automobile heat exchanges, such as a radiator and a heater core. A brazing sheet is made of an aluminum alloy (hereinafter simply referred to as an “Al alloy”) including a core layer and a brazing filler layer and/or a sacrificial layer disposed on one or both sides of the core layer. The typical tube for use is one produced by winding, in a tube shape, a brazing sheet with a three-layered structure mainly made of an Al alloy (brazing filler layer/core layer/sacrificial layer) with the sacrificial layer placed on the inside.
In general, an electric resistance welding method is used to form the brazing sheet into a tube. That is, in order to form the brazing sheet into a longitudinally tube shape, a strip-shaped brazing sheet is permitted to pass through a number of forming rolls and then formed gradually into a rounded tube shape, thereby producing an open pipe. Next, a coil positioned away from the outer periphery of the open pipe is used to heat only the vicinity of an open part at a high temperature by high-frequency induction heating. Thereafter, the open pipe are pressurized from both sides thereof by a pair of squeeze rolls disposed vertically to pressure-weld both ends of the open pipe, thereby closing the open part to produce a continuous tube.
In recent years, tubes made of aluminum alloy brazing sheets for use in heat exchangers of automobiles and the like have been developed to achieve high strength, high corrosion resistance and so on, in order to decrease the weight and thickness of materials. However, with decreasing thickness, welding defects tend to increase in the electric resistance welding for the tube. Thus, the improvement of the pipe productivity in the electric resistance welding, that is, electric resistance weldability, is strongly required.
As measures for improving the electric resistance weldability, various techniques have been conventionally developed. Patent Document 1 supposes that the degradation of electric resistance weldability is caused by an inappropriate area occupancy rate of Zn—Mg based intermetallic compounds on the surface of a sacrificial layer, and thus proposes that the area occupancy rate of Zn—Mg based intermetallic compounds is restricted to 1.0% or less. Patent Document 2 supposes that the degradation of electric resistance weldability is caused by the inappropriate number density of Al—Cu based intermetallic compounds in the core layer, and thus proposes that the number density of the intermetallic compounds is restricted to 10 pieces/μm2 or less.